Lead-on-chip (LOC) assembly technology represents a method for control of package size by eliminating the distance between the edge of the die paddle and the tip of the lead finger. Conventional assembly methodologies employ silicon chips that have aluminized bond pads located along the periphery of the die. Die bonding involves adhesive dispense onto a leadframe die paddle followed by chip pick-and-place. The adhesive, which is typically a silver filled epoxy resin, is subsequently cured off-line in a box oven or cured in-line in a snap cure oven. Following curing, conventional gold wire bonding is performed.
Lead-on-chip assembly methodology differs in several important ways from conventional assembly methodologies. Indeed, LOC assembly methodology requires modifications to the assembly equipment, leadframe design and aluminum pad layout on the silicon chip. The modification to the silicon die is probably the most significant factor since mask modifications are required for both the aluminum metal and passivation layers. In addition, lead-on-chip assembly requires that the pads on the silicon die must be relocated from the die periphery to the central portion of the chip. Furthermore, the lengths of the lead fingers of the leadframe employed for the preparation of lead-on-chip assemblies are extended.
The preparation of lead-on-chip assemblies described in the prior art does not employ conventional die attach to the backside of the silicon die. Instead, a thermoplastic tape is bonded in a window frame configuration to the underside of the leadframe. Typically, the leadframe supplier applies the thermoplastic tape to the leadframe. The tape typically has three layers consisting of a polyimide film, sandwiched between two thermoplastic films. The polyimide film provides rigidity to the tape and alpha particle protection to the active structures of the silicon die.
Bonding of the components involved in the preparation of an LOC assembly involves heating of the leadframe and/or silicon chip to a temperature at which the thermoplastic film softens. The leadframe is aligned over the die and the two are brought into contact. As the thermoplastic film softens, it flows slightly and bonds to the passivating layer of the silicon die.
It would be desirable to be able to carry out high-throughput LOC assembly without the need to use the thermoplastic tapes employed in the art. In addition, it would be desirable to be able to selectively apply adhesive to the die and/or leadframe prior to bonding the two components together. However, commercially available equipment for conventional die bonding and customized equipment for tape-based lead-on-chip assembly cannot be directly retro-fitted to accommodate paste-based LOC assembly. Accordingly, there is a need in the art for the development of apparatus and methods for the preparation of lead-on-chip asemblies employing paste-based adhesives.